In general, it has been recently proposed to exchange various kinds of metallic parts of motorcars for plastic products and put into practice because plastic materials are lighter than metallic materials. Among plastic products, polyamide resin products are excellent in toughness, heat resistance, oil resistance, etc., so that they are often applied to parts, which are used at a high temperature, such as a cylinder head cover, an oil pan. From the viewpoint of noise control of motorcars, these plastic products are expected to provide significant improvement of vibration control and sound insulation. They are thus useful for reasons other than their light weight. However, there is a problem in which it cannot be expected for polyamide resin products to effect the vibration control, because the vibration controlling performance due to the viscoelastic properties of plastic materials is seriously lowered under a high temperature condition near to 100.degree. C. for example in a cylinder head cover or an oil pan. This is due to the fact that on one hand plastic materials in general have highly viscoelastic properties and expected vibration controlling performances but, on the other hand, they depend on temperature so that their vibration controlling performances are best at about the temperature at which the loss tangent of dynamic dispersion due to the glass transition of plastic material (hereinafter referred to as "tan .delta.", cf. the note of Table 1) is maximum, and in general remarkably lowered at a temperature in the range of practical uses. For instance, in common polyamide resins such as nylon 12, nylon 6 and nylon 66, the maximum value of tan .delta. due to the glass transition appears in general at about 50.degree. C. and the value of tan .delta. becomes very small at higher temperatures. Thus, the lowering of the value tan .delta. is a main reason why a great vibration controlling effect is not expected under a condition for the usage at high temperature.
As a method to improve the above undesired property of polyamide resin, it has been proposed in U.S. Pat. No. 3,472,818 to shift the glass transition temperature of polyamide resin to high temperature, for example, to copolymerize with a small molecular monomer, such as aromatic monomers or cyclohexane ring containing monomers. However, any substantial effect could not be expected for such a method because of problems such as the reduction of workability, the lowering of vibration controlling performance at low temperature, etc. From the viewpoint of keeping vibration controlling characteristics in a wide range of temperature, the most expected method is to modify polyamide resins by providing multicomponent-multiphase polymers by blending polymers or grafting. However, polymers which can be blended to polyamide resin without any great lowering of strength, etc., are limited and these polymers such as nitrile rubber, acid-modified EPDM, ionomers, etc., are known, but the glass transition temperatures of these are less than 0.degree. C. and a good improvement is not found out at high temperature. To further disadvantage nitrile rubber lowers the heat resistance, and both acid-modified EPDM and ionomers decrease the heat and oil resistance. The grafting method as described in U.S. patent application Ser. No. 580,513, filed on May 23, 1985, U.S. Pat. No. 4,505,716, also has been not satisfied because polymers capable of being grafted with polyamide resin are limited and an increased cost is results in such a method.
Therefore, the present inventors have made various studies to produce a polyamide resin composition having an excellent vibration controlling performance within a wide temperature range from low up to high temperatures and excellent strength characteristics. This object can be achieved by the present invention which will be described hereinafter in detail.